Andreev and normal reflections in gapped bilayer graphene–superconductor junctions

We study the Andreev and normal reflection processes---retro as well as specular---in a bilayer graphene--superconductor junction where equal and opposite displacement fields are applied for the top and bottom layers to induce a band gap. By employing the Dirac-Bogoliubov--de Gennes equation for the gapped bilayer graphene--superconductor junction, we calculate the reflection probabilities within the scattering theory approach. The subgap conductance, calculated in the framework of Blonder-Tinkham-Klapwijk formalism, shows the contribution from the Andreev retro reflection (specular reflection) when the applied bias voltage is below (above) the Fermi energy. Notably, both retro and specular reflections are modified in the presence of the displacement field, and the retro-to-specular crossover gets amplified when the displacement field is relatively small. They can be further tuned to either specular or retro Andreev reflection by adjusting the Fermi energy. Furthermore, our study reveals the simultaneous existence of double Andreev reflections and double normal reflections when the displacement field becomes comparable to the interlayer coupling strength. The existence of the normal retro-reflection process in a bilayer graphene--superconductor junction is a finding which shows a distinctive feature in the conductance that can be experimentally verified.